Paid Courses

PayPal

PayPal enrollment allows users to pay for courses and then be automatically enrolled.

https://roseathemes.com/monocolor/1/enrol/index.php?id=5roseathemes.com

Check how the UI for paid courses looks like.

PayPal enrolment - MoodleDocs

Official Moodle Documentation.

Stripe

Moodle plugins directory: Stripe

Stripe Plugin to download.